Noise suppression circuits



T. P. KINN NOISEv SUPPRESSIGN CIRCUITS Jan. 9, 1940.

Filed Sept.- 16, 193'? l mmi Nm): A \m.

V' Q N kk 9%* MAN INVENTORv WITNEssEsf m M P. e r m ATTORNE PatentedJan. 9i, .1940

. UNITED l STATES NOISE SUPPRESSION. `v'cnwm'rsf Theodore P. Kinn,lSpringfield, Mass., assigner. to

Westinghouse Electric & Manufacturing Com.- pany, East Pittsburgh,

Pennsylvania lApplication september 1e,1e3"1,-seria1Noname@ ff,

izclaims.v (dise-20) g ThisI invention relates to radio receiving sys-4tems, and more particularly to receivers of the type wherein. the`sensitivity' of the amplifier stages isautomatically varied in inversepropor- 5 tion with the strength of the received carrier wave. D f 1 Anobjectionable Vfeature of radio receivers equipped with automatic volumecontrol resides in the noise appearing in the loud speakerwhen v the setis tuned from one station to another. This noise is especially strongbetween kstations where no carrier `wave is present, which 'woulda'ctuate the automatic volumey control and thereby reduce thesensitivity of the receiver. The interfering noise is attributed to thefact 'that in receivers of the above type the amplifying tubes at nosignal inputV are operated to the full extent of their amplifying power,and in this state of maximum sensitivity will amplifymi- Y2() nuteelectrical disturbances as well as'even slight variations of currentappearing within the tube. The latter condition is usually referred tovas lthe shot-effect, and results in a steady highfrequencyhiss in thetube output due to the fact that the electron `emission from the cathodeis not perfectly regular but 'spasmodic in charac` ter. A

From the foregoing, it 'is readily seen that upon the absence of thecarrier wave to which the'receiver is tuned the automatically readjustedsensitivity of the set results in objectionable'inter-channel orback-ground noises. This v'may beexperiencedvnot only in the tuning ofthereceiverbut also when atmospheric conditions cause fading of thesignal. l

Jn order to eliminate the above disadvantage, various circuits have beendesigned in which auxiliary devices performed the silencing function.Some were in the form of manually operated' 40 switches, so-calledmuteing buttons which were pressed down at the time the tuning dial wasmoved and thereby short-circuited certain portions of the signalamplifying channel. Others employed auxiliary vacuum tubes per- 45`forming a trigger function in the audio amplifier circuit. Theauxiliary tu'be was subjected to the automatic volume control potentialin such manner that upon the absenceofa control potential,

, ,the heavy vcurrent drawn by the tube disbalanced a iixed state ofvoltage distribution necessary for the operation of the tubes in theaudio amplifying system. The latter'type, known as the quelch or QAVCsystem, while automatic in operation requires' the use of an extra tubein the ,55 receiverwhich contributes nothing to the primary function` of`the v acuum tubes in the-set. Furthermore, it complicates 'thecircuitinasmuch as several components to the receiver proper must be added' in`order yto obtain an operative arrange- The `primaryobject-of thisinvention is to 'eliminate inter-channel .-or back-,ground noises .in a.

radio receiver, by providing/a circuit arrangement which requiresno'additional tubes 1 1o ra1 1 x iliary' devices to perform the statedobjective. 1

It is a particular feature of ythis invention. that, the meansprovidedgfor suppressing.inter-channel noise,`is simple inoperationand-easily applicable to al1-types of radio receivers havingautomatic sensitivityy controls. Y.

A further';;advantage `resulting from the use of the proposed meansinaccordance with this invention is that existing receivers mayA easilykbe modified to perform in the mannenstated without requiring anymajorchanges or adjustments in the receiver circuit.

Additional objects and advantages will be apparent from the'followingdescription of the invention, dened in particular-ity byy the f appendedclaims land takenin connection with the accompanying drawing, in whichthe invention is illustrated diagrammatically in the-intermediatefrequencyamplifier-portion of a radio receiver.

Referring to the drawing, the single figure showsa portion of a radio,receiver, namely, `the signal-translating jchannel rof the intermediatefrequencyamplier in a superheterodyne rtype` receiverthe `,detectorstage and ythe rsucceeding` audio Afrequency stage. Therotherparts' ofthe circuit preceding the intermediate frequency amplier have beenjomittedv for thesake of simplicityf,inasmuchv as the invention hereindescribed may advantageouslybe employed to one high-frequency amplifyingstage. It is assumed, ofcourse, vand anybody skilled in the art will ,40readily recognize, that an` oscillator and first detector stage willprecede the arrangement shown in the dravvingqto-complete asuperheterodyne receiver, `andan additional stage of audiofrequencyamplication may also `be needed ifa 4 5 loudspeaker Mis l.tobel operated. lurtlfierrnore, Ait will be 'clear as thedescriptionyexplains .the inventionin itstechnical details, that it need vnot berestricted toa-superheterodyne type of receiver` but will operate in anyradio frequency 50 st age as long as. there is an (automatic controlresponding to the strength of the carrierlwave. The circuit shownessentially consists of two high-frequency amplifying stages including4the vacuuml-,ubes'V `l 4r'and @..adetector stage'.includinie,r

vgrid 6, a screen grid I and suppressor grid 8.

The latter in some tubes is tied to the cathode withiny 4the tube .and.2in others, the connection may beieffected outside of the tubeproper.The input circuit vto each pentode tube comprises a transformer Ivof thedouble-tuned type, generally employed in intermediate frequency, orcommonly referred to as IF stages. In series with each cathode is aresistor II which is so proportioned as to effect thenecessary voltagedrop vby the anode current of the-tube for the minimum operating biasrequired in accordance with the type of tube used. Each resistor II isby-passed by a condenser I2."`Following the input circuit of each'stage,the return of the secondary coil of each transformer I 0 connectsthrough-resistors I3 and l'ltothe load resistance I6 of the detectortube' 3 vand thence to the catho'de "-35 thereof which is .connected toground. An additional Aresistor I4`for filtering purposes* is includedin this circuit and by-pass condensers 23, 25 `and `26 providelow'impedance paths to ground forhigh-frequency currents.

v"The circuit above referredto comprises the automatic'volume controlportion of the receiver together withthe detector tube 3 which is thesource of automatic volume control, or using its accepted abbreviatedform,`A. V. C. voltage. The 'detector tube 3 operates as a diode andrecties the 1'high-frequency current in the secondary Winding of thetransformer I connected between its anode 36=and cathode 35 in serieswith the Iload resistance I6 previously referred to. Due to therecticationf of the tube 3, a voltage drop is' effected bythecurrent/flowing in the resistor ist Yits V'corresponding transformer IE.'22 by-pass the output circuit to ground. The

l'w'vhich canI` be resolved into two components: one the modulatedcomponent of the high-frequency current which istransmitted through thecoupling condenser' 21 to the grid resistor I1 of the audio amplifier4'and comprises the audio frequency; and a unidirectional componentwhich at the point where the resistors I5, I6 and the secondary Windingvof the transformer I0 join is negative with respect to the'cathode 3,5and proportional in magnitude to the high-frequency `voltagesulpplied`by the amplifying tubes to the detector tube `3. "Iherefore, when thereisno currentflowin the resistor I6, thegrids 3 are effectively at groundpotential and biased only by the minimum bias derivedfrom the resistorsIl. "This is the case when there is no signal voltage tothe detectortube. I-Iowever, when the 'detector'tube' is supplied with signalvoltage and rectifies the latter, the grids' 6 willbe biasedincreasingly negative bythe rectifiedcomponent of the signal voltageappearingy across the resistor I6. This is essentially the A, V. C.action and willbe further exemplified in describing the Aoperation ofthe circuit as a whole.l v

"The output circuit of eachamplier tube includes the anodes 9 and thevprimary Winding of Condensers firstY IF amp-liertube plate return isconnected 4directly to the power 4supply schematically shown bylter'condensers 32, 33,-iilter reactor 34 and the voltage divider 3l. Thesecond IF tube plate Yreturn includes a series resistance 20 which maybe: short-circuited'by contacts42 and 43 of the switch 4G. The screengrid 'I of the tube I bypassed'by condenserfZI lreturns directly'to theance shown by an audio transformer 33. 4arrows'of `the secondary windingindicate that tap 45 of the voltage divider 3l, Whereas the screen grid'I of the tube 2, by-passed by condenser 2I includes a series resistanceI9 in the return lead to the tap 45, and may be shortcircuited bycontacts 4I and 44 of the switch 40. The two resistors I9 and 2U form animportant partin the operation of the-system to silence the receiverupon absence of signal voltage, and more will be said thereof later.

-The rst audio amplifier tube 4 is' connected in a conventional manner,having a bias resistor I8 connecting the cathode 38 to ground and theplate circuit includes a suitable output imped- The the outputcircuitmay terminate in a` succeeding amplifier or a suitabletranslating device not shown here. Similarly the arrows afxed to the A.V.,C. potential supply lead, the screen and plate supply conductors,indicate that preceding `portions of` the receiver may be supplied fromthe same source for all operating potentials. The filaments forheatingthe cathodes of the various tubes and the source of heater current havebeen omitted in order to simplify the drawing. It is well known in theart that the heaters may be supplied from any suitable alternatingcurrent source, such asI a transformer, for instance.

Before'describing the operation of the receivingy system embodying theimprovements in accordance with this invention, a'clearer understandingthereof will be had by dening the term normal order of potential whichis to signify here the proportion of the voltages supplied to avacuumtube in order that it may function in the manner intended, for exampleas an amplier, In other words, by normal order is meant the respectivemagnitudes of operating voltages' required for a particular tube for itsprincipal electrodes. In a screen grid tube, for example, to operate asan amplifier the normal order of potentials is a higher positivepotential lon the anode than on the screen, both potentials being takenwith respect to the cathode. Now, let` the vacuum tubev 2 be consideredfirst in connection with its associated components but apart from therest of the system. The circuit elements are so proportioned that withthe minimum" grid bias potential derived from resistor II bythe platecurrent flow, the operating potentials on the screen 1 and anode 9 willbe subciently lowered due to the voltage drop in the series resistors I9and 20 and in such proportion as to upset the normal order of potential,the screen becoming more positive with respect to the cathode than' theanode. This causes blocking of the tube for signal transmission. Statedin another way, with maximum sensitivity setting of the tube, the platecurrent causes a voltage drop of such magnitude across the resistorturn, decreases the currents in both resistors I9 and 20. Thus, anegative increase in grid bias of the tube 2 causes the average platevoltage to rise producing thereby an increased mutual conductance withconsequent increase of amplification. Considering the rst I. F.amplifier tube, which is conventionally connected and includes no meansvaffecting its operating potentials, the

mutual :conductance increases with decrease ef biasv potential and 'atno signal -input to the 'systemi,f as fior instance, when the setv isItuned between adiacent broadcast channels, the amplification is atmaximum. Whereas, in the pres# ence of the lsignal with 'increased gridbias, `its amplification decreases until equilibrium "isreached betweensignal transfer and control potential. Bea-ring in mind thehdifferentiallfunction of the rst and second amplier tubes, it will be seen 'that inthe `presenceof a carrier Waveland due to the A. V. C. action, 'bothtubes become operative and normal reception Yis obtained. On the otherhand, in the absenceof a carrier Wave while the rst amplifier tube isatmaximum gain the second amplifier tube is inoperativefand no signal inthe form of interfering noises" Aand hisses is transferred pastv thestage preceding the detector.` The circuit constants and the resistancevalues `for resistors l and Zare so proportioned that' While there is notransfer to the detector tube of the ampliiied electrical disturbancesthrough the tube 2, the signal voltage of a broadcastingqsta# tionconsidered local to the receiver, having a carrier intensity greatlyinexcess of the amplied value of the above disturbances, Awill passthrough by capacity effect'and in suflicient'ma'gnitude to be acted uponby the detector. The resultant A. V. C. bias offsets the inoperativestate of the tube `2 and normal reception is obtained. f

Y- The simple switch 40' is included Afor the purpose that, if fullsensitivity of operationisde-y sired at alltimes as for instance whenextremely weak signals are to be tuned in, the resistors I9 and Zbeshort circuited. 7

It has also been observed in practice that in some receivers it isadvantageous to include the noise suppression feature in more than oneVstage.

From the above description, it* will be 'apparent that the onlychangenecessary to a standard receiver embodying automatic volume control-inorder to obtain the advantages of the invention, constitutes the mereaddition of a resistor in the screen grid and plate lead of theamplifier to the voltage supply source, so proportioned that underconditions whichwould normally produce maximum amplification in thisstage, the plate and screen voltages drop to a relatively low value, andwith the rplate voltage at a lower value than the screen voltage, thetube becomes inoperative as an amplifier.

Through the addition of these two elements tov an otherwise standardcircuit, the objects of this invention have been attained andsuchresults obtained which in accordance with present practice necessitatesthe addition of another electron discharge device to the circuit,together with such circuit elements which are normallyv required toenable an electron discharge device t0 function in its intended manner.

While the invention has been described in i charge device having acathode and a plurality of principal electrodes, means for applying anormal order of potential to said principal electrodes `principalelectrodes causing inoperativeness said device.

withY respect to lsaid'` cathode 'duringr periods of signalfamplication,means responsive to va'ria tiens Ain the intensity of theincoming signalWave" for'maintaining the volume level of said amplifier substantiallyconstant during amplification of signals, and means for reversing thenormal Aorder of ootentialsto certain of said principal electrodes'torrender said amplifier substantially non-operative during no signalperiods.

2;4 Incombination, means for amplifying'highfrequency'energy comprisingan electron dise'. charge device having a cathode and a pluralityofprincipal electrodes, means for applying a f normal vorder of potentialsto said principal elec-i' trodes Awith respect to said cathode duringlperiods ofV signal amplification, means responsive to 'variations of theincoming carrier wave for varying the amplification of said amplifiervin accordance with said variations in carrier intensity, and means forreversing the normal o`r-y diate electrode `during no signal periods.

4. In combination, anfampliiier including jan' electron discharge deviceIhavingA a cathode,l an anode andan intermediate electrode,means forapplying positive potentials tol said anode and intermediate electrode,the potential on said ancde being of aliigher order than that Qn saidintermediate electrode during amplification `periods, means formaintaining the Vvolumey levell amplification of signals,`and means forreducing the voltage oi said anode to a value below that applied to saidintermediate electrode during no signal periods.

5.y In combination, an electron discharge device'having a cathode, anranode and an inter. mediate electrode, means for applying positivepotentials to said anode and intermediate electrode, the potential ofsaid anode being of al higher order than that of said intermediateelectrode during amplification of signals, a voltage dropping device incircuit with said anode and having a value of impedance sufhcient todrop said anode potential below that of said intermediate electrodepotential when said amplier is otherwiser adjusted for maximumamplification.

6. In'combiriation, an electron discharge device having a cathode andprincipal electrodes, means for applying a normal order of potential to'said principal electrodes with respect to said cathode essentialr tothe operation of said device during periods of said signalamplification, means for applying a'biasing potential tor said amplifierto from a predetermined maximum value and means of saidy amplifiersubstantially constant during I operative upon loss of said bias forreversing said normal order of application of potentials to said 7.Incombination, an electron discharge device to said anode andyintermediate electrode, the

potential on said anode being of a lower` order than that on saidintermediate electrode during the absence of signal energy in the inputcircuit .5, of said amplifier, and means responsive to the applicationof signal energy to the input circuit of said amplifier for causing saidanode potential to rise in value above that of said intermediateelectrode potential.

8. In a signalling system, a signal-translating channel includingamplifiers producing an amplied signal output, means responsive to saidamplified signal output for producing a unidirectional voltage, meansfor controlling the ampliiication in certain of said signal ampliers ininverse proportion with said unidirectional voltage, and means forapplying said unidirectional voltage to a control element of one of saidampliers to control the responsiveness thereof in direct proportiontherewith, whereby said signal amplifier is rendered inoperative 'whenthe signal input applied to the other of said ampliiiers decreases belowa certain value.

9. In a signalling system, a signal transmission channel including anoutput coupling element, a pentode amplier coupling said output to arectier producing a rectiiied signal voltage, said pentode ampliiierhaving in its plate lead a high resistance which reduces its averageplate voltage to a point of inoperativeness, means for rendering saidpentode amplifier operative upon signal transmission of a predeterminedintensity by making its control grid more negative in accordance withsaid rectiiied voltage and thereby increasing its plate voltage. n

10. In a signalling system, a pentode amplier,

a rectifier for producing a unidirectional voltage dependent on theamplification in said pentode amplifier, a connection from a point insaid recti- 40 fier circuit to a control element of said pentodeamplifier for automatically adjusting the control grid bias, a highresistance in the plate lead of said pentode amplifier proportioned tocause said amplification to increase in predetermined relation toincrease of control grid bias, whereby in response to a Variation ofinput to said signal ampl-ierl said unidirectional voltage is caused tovary with respect to said control grid bias substantially in accordancewith said predetermined relation. i

11. In a signalling system, a signal 'channel producing a signal output,a pentode amplifier coupling said output to a circuit including arectiier for producing a unidirectional voltage dependent on said outputand on the amplication in said pentode amplifier, a connection from apoint in said rectifier circuit to a control element of said amplier forcontrolling said output in response to said unidirectionalpvoltage, ahigh resistance in the plate lead of said pentode amplifier proportionedto cause said amplii'lcation to increase in predetermined relation toincrease of control grid bias, and means for preventing saidl amplifierfrom responding until the output from said rectifier exceeds apredetermined value, whereby in response to a variation of input to saidsignal amplifier said unidirectional voltage is caused to vary withrespect to said control grid bias substantially in accordance with saidpredetermined relation.

l2. In a signalling system, a signal amplifier producing an amplifiedsignal output, a pentode amplifier coupling said output to a rectifierproducing a rectied signal Voltage, said pentode amplifier having in itsplate lead a high resistance which reduces its average plate voltage toa point of inoperativeness, means for reducing the ampliiication in saidsignal ampliiier in accordance with said rectified voltage andsimultaneously rendering said pentode amplifier operative, by making itscontrol grid more negative and thereby increasing its plate voltage, inaccordance with said rectied voltage.

THEODORE P. KINN.

